map editor blender game engine
i would split the map editor up in two parts, Terrain editor and Prod editor.

For the Terrain i would do it with textures maps, so when you edit the terrain you are painting on Bump,Normal,Spec,Diff and so on.

For the prod you need to make a inventory like menu, and then Edit object "add/remove" in a RTS like way.

This might be a quite big project, but if you split up the problems in smaller pieces i think it can be done, and it will make it way more convenient for other ppl to help you with it.